1. Field of the Invention
The present invention relates to an armature having an armature core with 72 slots, and a three-phase two-pole armature winding wound in two layers and housed in the slots.
2. Description of the Related Art
In a large-capacity dynamo-electric machine, an armature winding is provided in two layers in slots with upper coil pieces and lower coil pieces provided in a laminated core, and the two layers of armature winding are connected in series to provide a high voltage, thereby increasing an apparatus capacity. However, as an armature winding rises in voltage, the thickness of a main insulator of the armature winding needs to be increased to withstand the voltage. As a result, the cross-sectional area of a conductor of the armature winding is decreased. This increases a current density and loss.
Particularly, in a machine adopting indirect cooling system for cooling an armature winding from the outside of a main insulator, a thick main insulator increases thermal resistance and temperature of an armature winding. Therefore, an armature winding is divided into two or more parallel circuits to decrease in voltage and main insulator thickness, while keeping an apparatus capacity, thereby increasing a cooling capacity with decreased loss. Particularly, in an indirect cooling large-capacity machine, it is common to increase the number of slots to increase the peripheral length of an armature winding to be cooled. Therefore, it is necessary to use an armature winding having three or more parallel circuits.
However, if a two-pole armature adopts an armature winding with three or more parallel circuits, it is difficult to generate the same voltage in parallel circuits. Therefore, a circulating current occurs generated among the parallel circuits, and increases loss in the armature winding. To decrease the loss caused by the circulating current, it is necessary to minimize the unbalance among the voltages generated in the parallel circuits. It is thus necessary to give special consideration to arrangement of coil pieces in each parallel circuit in each phase belt.
An explanation will be given on an example of improvement in arrangement of coil pieces by referring to a developed perspective view of an armature winding in FIG. 7, showing a part for one phase. FIG. 7 shows an example of an armature winding with four parallel circuits applicable to a dynamo-electric machine with three phases, two poles and seventy-two slots, according to U.S. Pat. No. 2,778,962 (hereinafter, called Literature 1). FIG. 7 shows a part for only one phase. It is however appreciated that parts for the other two phases are obtained by displacing the configuration of the armature winding phase of FIG. 7 by 120° and 240°, respectively.
In FIG. 7, when parallel circuits are indicated by numbers 1 to 4 (parenthetic numbers 1, 2, 3 and 4), twelve upper coil pieces 15 and lower coil pieces 16 in a first phase belt 17 are numbered 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2 and 1 sequentially from the center of a pole, and twelve upper coil pieces 15 and lower coil pieces 16 in a second phase belt 18 are numbered 3, 4, 4, 3, 4, 3, 3, 4, 3, 4, 4 and 3 sequentially from the center of a pole, thereby decreasing a voltage deviation (an absolute value of deviation from an average phase voltage) in the parallel circuits and a phase difference deviation circuits (a phase angle deviation from an average phase voltage) in the parallel circuits.
To realize the above connection, in FIG. 7, fourteen jumper wires 20a are provided for each phase at a connection side coil end 19a, but no jumper wires are provided at a coil end 19b opposite to the connection side.
As for the voltage deviation and phase angle deviation in parallel circuits, U.S. Pat. No. 2,778,963 (hereinafter, called Literature 2) indicates that a reference value of voltage deviation is 0.4% or lower is, and a reference value of phase angle deviation is 0.15° or lower. However, in the Literature 1, the voltage deviation is 0.12% and the phase angle deviation is 0° in the parallel circuits, which are well balanced compared with the above reference values, and are enough effective to decrease a circulating current.
The connection method disclosed in the Literature 1 is electrically suitable with the generated voltage deviation reduced to a minimum, but is mechanically complex in the structure. Namely, to configure the armature winding shown in FIG. 7 according to the Literature 1, it is necessary to provide fourteen jumper wires 20a for each phase at the connection side coil end 19a, to connect the upper coil pieces 15 and the lower coil pieces 16. Connection of the jumper wires 20a is an additional work, and it is important to ensure the insulation and fixing strength of the jumper wires 20a. There are twenty locations per a phase to connect the upper and lower coil pieces 15 and 16, except a location to connect a lead-out connection conductor 21, at the connection side coil end 19a. Fourteen locations per a phase among these twenty locations are connected by the jumper wires 20a. As the jumper wires 20a are tightly arranged with small spaces, the jumper wire connection work is uneasy, and it is difficult to ensure the insulation and fixing strength of the jumper wire owing to interference between the jumper wires 20a and between the jumper wire 20a and lead-out connection conductor 21.